It's called "I use the software, I already want to improve the software I'm using, so after I improve it I'll contribute the improvements I've already made to the broader community."

Granted, I myself have been guilty of not giving back to the open source community this way in the past, but I won't pretend that was reasonable or ethical of me!

edit: after reading some commemnts, i realize i may have meant to say "free software" instead of "open source"